In the semiconductor industry, it is common to pre-align a semiconductor wafer as part of readying the wafer for processing. The pre-alignment operation conventionally includes locating and precisely positioning the geometric center of the wafer. Once this operation is complete, the wafer is then placed in a selected orientation related to the orientation of its crystalline structure. This pre-alignment procedure is commonly completed as a separate operation before the wafer is transported to a processing apparatus for providing a desired finishing or processing step.
Conventionally, a robot removes the individual wafers, one at a time, from a transportation carrier and places them at the pre-alignment station. After the wafer is aligned, i.e., after the wafer is properly oriented and its geometric center is located, the wafer may be placed back into the transportation carrier or in a processing carrier in the pre-aligned condition.
It will be appreciated from the above that precise pre-alignment not only is desirable but can be a major factor in determining ultimate reliability of the integrated circuitry produced on a wafer. Many operations require very accurate alignment, and accurate pre-alignment reduces the mechanical and operational constraints in achieving such alignment.
FIG. 1 illustrates a conventional pre-aligner 10. This pre-aligner 10 includes a “light bar” as a light source. A light bar 12 typically consists of a row of multiple LED's 13. Using a light source which consists of multiple, individual LED's 13, has several disadvantages. For example, there is often a mismatched brightness between individual LED's 13. The mismatched brightness may cause a variation of the image intensity focused at the CCD 14. A non-uniform image intensity may cause a non-linear detection of the wafer's edge 16. Other problems may surface as the pre-aligner 10 ages. Specifically, the brightness of the individual LED's 13 will fade at different times. This fading will eventually lead to an uneven or non-uniform illumination. Accurate centering of the wafer requires even or uniform illumination. Therefore, a pre-aligner 10 using a conventional “light bar” 12 requires calibration in the factory which must be repeated every time the illumination becomes non-uniform. A diffuser 18 is commonly used to even out the distribution of light and minimize any effect stemming from the non-uniform illumination. The diffuser 18 is also used to control the total amount of light emitted by the array of LED's 13. Further, because the light emitted from the light bar 12 is uncollimated (contains light rays that are not parallel to each other), a camera lens 20 is commonly used to focus an image from the wafer 11 onto the CCD 14. Such a lens 20 is expensive.
Although a pre-aligner 10 using an array of LED's 13 as a light source commonly provides high accuracy, there are several disadvantages. By way of example only, it is necessary to perform intricate factory adjustments, tune the diffuser 18 to achieve uniform illumination from the multiple LED's 13. Additionally, since the light is uncollimated as it passes the wafer edge 16, the pre-aligner 10 is very sensitive to vertical runout of the wafer 11. Because of the non-vertical light, vertical runout of the spindle, which moves the wafer edge 16 up and down, also moves the shadow along the CCD 14. This up and down movement is indistinguishable from the shadow movement caused by horizontal wafer movement, which is the variable being measured.
In addition to electrical disadvantages, there are also mechanical limitations to the pre-aligner 10. The lighting assembly is vulnerable to damage. The light bar 12 mounts in a housing 18, which is suspended above the body 22 of the pre-aligner 10. Thus, the housing can be easily bent during shipping and handling, or by a collision with a robot arm. Additionally, because the light bar 12 is mounted above the CCD 14, electrical wiring must be routed out of the main body 22 of the pre-aligner 10 and up into the optical housing 18.
Some conventional pre-aligners, such as the pre-aligner 50 shown in FIG. 2, use light emitted from a single LED 52 to cast a shadow of the wafer edge 54 directly onto a CCD 56. Using a single LED 52 mounted above the wafer 51 (i.e., to a printed circuit board 58) and the CCD 56 has several disadvantages. For example, the pre-aligner 50 is very sensitive to vertical mount of the wafer 51. Similar to the light housing 12 shown in FIG. 1, the light emitted from the single LED 52 as it passes the wafer edge 54 is uncollimated. Thus, movement of the wafer up and down between the LED 52 and the CCD 56 moves the shadow casted onto the CCD 56.
Using light from a single source also has a magnification effect. As a result of the non-vertical light emitted from the LED 52, the movement of the shadow cast onto the CCD 56 is greater than the movement of the actual wafer edge 54. This exacerbation effect must be compensated for by added complexity in the software that determines the location of the center of the wafer.
The single LED 52 is also an inefficient use of light energy. Light emitted from the LED 52 fans out to a circular pattern by the time the light reaches the CCD 56. Thus, a majority of the light actually falls directly on the CCD 56 and not the wafer 51. Thus, the LED 52 must be closely located above the CCD 56 to provide adequate illumination. However, a LED 52 that is closely positioned to the wafer causes a spread of the shadow of the wafer edge 54 on the CCD 56. This effect is greatly exaggerated in FIG. 2 for illustration purposes. The “spread shadow” effect increases the uncertainty in determining the actual radius of the wafer edge 54 from the center of the spindle (not shown). As shown in FIG. 2, erroneously, the CCD 56 will measure the wafer edge 54 by a distance d. Further, as the LED 52 ages and its brightness diminishes, the sensitivity of the pre-aligner 50 will change over time.
Another conventional pre-aligner, such as pre-aligner 80 shown in FIG. 3, uses a laser diode 82 as a light source. A pre-aligner, which uses a laser diode 82 as a light source, has several advantages. With a single light source there is no possibility of a non-uniform light source. In the case of using a laser diode 82 as a light source, the laser circuit produces the same light output throughout the life of the product. Thus, no calibration is necessary to maintain an even or uniform light source. Additionally, because the light passing the wafer edge 84 is vertical, vertical runout of the spindle, which makes the wafer edge 84 move up and down, does not move the shadow along the CCD 86 in response to this runout. Thus, measurements of the movement of the shadow are more accurate. In addition, the vertical light path makes the shadow on the CCD 86 move exactly the same distance as the wafer edge 84.
In operation, light from the laser diode 82 naturally fans out in a circular pattern. The cylindrical lens 88 focuses the light into a narrow stripe by allowing the light to continue to diverge along the long axis of the CCD 86, greatly reducing the divergence of the light rays perpendicular to the long axis of the CCD 86. The light then encounters the two spherical lenses 90a and 90b, which are intended to refract the light to follow a vertical path past the wafer 81 and onto the CCD 56.
However, there are several disadvantages of a laser diode pre-aligner system similar to the pre-aligner 80. For example, the coherent nature of laser light results in speckles, or dark and light spots, on the CCD 86. These dark spots cause added uncertainty in the location of the shadow edge. Additionally, the laser diode 82 and associated regulating electronics are relatively expensive. Further, the narrow angle of light spread from the laser diode 82 requires that the light housing 92 be substantially taller than other optical systems (e.g., LED's). Similar to the previously mentioned pre-aligners, the laser diode 82 is mounted above the CCD 86 and therefore, electrical wiring must be routed out of the main body 94 of the pre-aligner 80 and up into the optical housing 92.
Accordingly, there is a need for a pre-aligner utilizing an LED as a light source, yet having the accuracy of a laser diode system. The present invention provides such a system.